TOPIC:

Martin wrote:

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@JR - what pinion will I need with this gear for a silvercan motor? Good acceleration/battery life. Not too worried about top speed.

I make that an 83T spur gear you have there - so the range of pinions that will fit should be 15T to 27T ... but I'm really skeptical about the usefulness of the larger sizes, so I'd go for the low end of that, so 15, 16, or 17T ... my guess would be the 15

Thanks JR. Did you count the teeth on the photo ?
So I guess a smaller spur gear (i.e. the tamiya one) would not be the right way to go then...

Also looking at wheels. I bought a set of Schumacher wheels for a few quid from ebay and the rears fit perfectly. The fronts take what I believe to be 10mm bearings. The Mardave stub axles are 4mm dia, so I need 10x4 bearings which seem to be difficult to find and expensive. I think the Mardave wheels take 8x4 bearings.
5mm id bearings are a lot easier to find so I am thinking about a way to convert to 5mm axles. This would also allow me to share wheels with the TRX-3.
The easiest way seems to be to swap the steering blocks with a model with 5mm axles (Tamiya?). Any suggestions?

Martin wrote:

Thanks JR. Did you count the teeth on the photo ?
So I guess a smaller spur gear (i.e. the tamiya one) would not be the right way to go then...

My information on gearing comes from www.planet-venom.co.uk/ , plus a bit of supposition ... if you manage to find the gear ratios page, there's a wide band of ratios for 48dp gears that "should" work, but IMO a spur gear of less than 82 teeth is going to be useless, as is a pinion of more than 18 teeth.

If you divide the number of the spur gear teeth by the number of spur gear teeth, then multiply that by 2.8572 (i.e. (spur T/pinion T) * 2.8572) that will give the overall drive ratio ... I have a 86T spur & a 14T pinion in my Cobra (so a 17.551:1 ratio) with a 19T motor, on grass that got a little on the hot side.

A 15T pinion on an 83T (yes, I did count them ) spur = 15.881:1 ratio, which may be a bit high, but I think a smaller pinion won't mesh.

TBH the move to 48dp gears is not a good one - it seems the belief that a slipper clutch is a "Good Thing" has overtaken the need to get the right gear ratio

I'm not really au fait with all this new-fangled stuff! I just remember that the Scorcher had a choice of two ratios and I preferred the one with 70T spur gear. No diff either and was still fun.
I had estimated very roughly the spur gear to wheel drive ratio at 3:1 (by turning the spur and watching the wheels), so 2.8572 makes sense.
AFAIK only one spur gear was available for the Cobra. Do you have one from a different manufacturer? (or do I?) Presumably anything will fit so long as the hole in the centre is the right size.
I am happy to do without the slipper. I will not be racing it and will not be running a super-hot motor. Slippers seem slightly illogical to me as they must waste energy. I would also like the opportunity to spin the rear wheels if I want!

Digging around in my bag-o-parts tonight I came across a couple of spare slipper gears for my Traxxas and there was an 84T one there.
I tried it on the Cobra slipper and it fits a treat!



I am assuming later Traxxas models have the same gear, so there should not be problems finding spur gears for the Cobra. Just need to get some more friction pegs now.